The paper manufacturing industry basically functions to procure a variety of generally fibrous cellulosic materials, physically as well as chemically break-down or alter such materials to a desired fibrous or pulp type consistency and reconstitute them in a dry web-like, form. For the most part, the raw material from which the pulp is generated is derived from tree wood, an estimated five million or more cords of such wood being harvested per annum in the United States. A broad variety of tree species are harvested for the purpose of pulp production, the procedure of harvesting generally providing for the cutting of tree logs into four foot lengths prior to the movement thereof into storage regions situate at mill facilities. Additionally the wood chip by-products of lumber manufacture are utilized as a raw material input to the paper forming process.
As a preliminary processing step, the harvested pulp wood logs are mechanically ground to form a preliminary pulp material, however, as a prelude to this procedure the bark portions thereof, i.e. that organic material extending outwardly from the cambium is removed. This "woodbarking" step is required inasmuch as bark has little fiber value, tends to overly consume the chemicals utilized in later processing steps and creates dirty pulp conditions. Generally, the bark represents from about twelve to twenty-five percent of the volume of rough wood, consequently, its disposition following removal from the pulp tree logs represents an industrial disposal problem. In some installations, the bark by-product is used in part as a fuel for later processing procedures, the fuel value of the material being about 9,000 BTUs per pound dry weight.
Conventionally, woodbarking is carried utilizing relatively elongate barking drums through which the logs are fed, debarking action taking place as the logs tumble and rub against one another and against the sides of the drums on their transit therethrough. The loosened bark drops through spaces or slots formed in the drum whereupon it is collected on a conveyor for removal to temporary storage. At some facilities, the bark is further shredded to a relatively uniform consistency.
Subsequent to the initial grinding procedures developing pulp wood chips and the like, a variety of manufacturing procedures treating the raw materials are employed. These procedures involve a number of unit operations such as gas absorption, evaporation, extraction and filtration to evolve a pulp product for delivery to the paper making stage. The pulp product is introduced to a stock preparation process where it is treated mechanically and, in some instances, chemically, by the use of additives, whereupon it is accepted for forming into a final paper web. As is well known, the multi-stage process generates a substantial waste representing a severe industrial disposal requirement. The waste ultimately exiting from the manufacturing process generally may be described as an effluent carrying very short fibrous matter, clay and other materials and having about a 95% moisture content. Usually, the effluent is dewatered through the use of settling tanks and/or centrifuges to accumulate as a sludge having about a 69% water content. Disposal of this sludge typically is carried out using land fill techniques.
From the foregoing it may be observed that typical paper mill installations will generate two principal waste products, bark resulting from woodbarking stages and sludge. While the sludge component is disposable through land fill arrangements, a less than desirable procedure, some uses have been found for the bark materials. Over the relatively recent past, industrial interest has been generated in conversion of the bark to a humus type byproduct useful in the horticultural industry. Generally the bark is transported to an available field whereupon it is deposited in windrows and subjected to periodic turning by typical farm implements over a given interval of time for example, about sixty days. During this interval, the bark is caused to undergo a fungal mold action, following which a period ensues wherein decomposition of the bark predominantly is carried out by bacteria active in a mesophilic phase at temperatures, for example, below 45.degree.-50.degree. C. The resultant material has been found useful and valuable, for example, as a potting soil as well as a conditioner for garden soils. The windrow form of treatment to achieve biodegradation of the material is considered somewhat crude, a reliable, uniform product not being guaranteed. Further, such technique is labor intensive, inefficient in and of itself and, additionally, not integrated with the overall industrial paper making process.